Hearing aids of the subminiature type, adapted to be worn immediately adjacent the auditory canal of a user, have previously been proposed--see German Utility Model DE-GM No. 83 23 464. The hearing aid there disclosed comprises a flat, hollow, cylindrical housing with a cover. The cover is rotatable to form an ON/OFF switch and volume control element, and is located adjacent an opening into the housing which is adapted to conduct sound from outside,ambient space to a microphone located within the housing. The cover, simultaneously, forms the switch, volume control and end piece for the housing.
It has been found difficult to use hearing aids of this type since adjusting the volume control without, at the same time, modifying the sound entrance path to the interior of the housing, is cumbersome. In order to rotate the volume control - cover element of the housing, fingers of the user invariably cover the opening leading to the sound duct. If the user, adjusts the volume of the volume control for a suitable amplification, and then removes the finger, the actual amplification, with the finger removed, will be excessive,and the sound pressure waves applied to the inner ear of the user will be much too high, and are highly annoying. Even continued experience with the failure of the volume control to accurately reproduce volume with the finger present or removed, will not always result in satisfactory amplification due to differences in placement of a user's finger when making the adjustment and/or differences in the frequency range, general sound or noise level or the like which differentially affect sound transmission to the microphone 25 path or through the user's finger.